FIG. 4 shows a circuit diagram for a conventional superhigh pressure control system applicable to water-jet type cutting apparatus (Japanese Patent Application Laid-Open No. 63-39799). The superhigh pressure control system comprises a booster 41 including a double acting hydraulic cylinder 42 having a piston P and plungers P.sub.1, P.sub.2 arranged at opposite sides thereof and fitted respectively in water-pressurizing plunger chambers C.sub.3, C.sub.4, and ports at ends of the plunger chambers C.sub.3, C.sub.4 which are connected in parallel to a water supply line 46 of a water supply pump 45 via suction check valves 43, 44, the ports being also connected in parallel via discharge check valves 47, 48 to a superhigh pressure water discharge line 49 equipped sequentially with an accumulator 50, a nozzle on-off valve 51, and a jet nozzle 52. A two-position directional control valve 54 for switching the reciprocating motion of the piston is provided between the respective ports at opposite ends of a cylinder chamber of the hydraulic cylinder 42 and a hydraulic pump 53. Air nozzles 57, 58 are fixed adjacent the jet nozzle 52 and in slightly spaced apart relation in the directions of movement (designated by arrows X, Y) of a moving carriage 55 on which is carried a material to be cut 56, the air nozzles being connected to an air pressure source 61 via on-off valves 59, 60. Relief valves 65, 66 are respectively disposed between the water supply line.46 and a water tank 62 and between a main line 63 for the hydraulic pump 53 and an oil tank 64.
When the hydraulic pump 53 is actuated with the two-position directional control valve 54 set to assume a symbol position V.sub.1, hydraulic oil is supplied to a cylinder chamber C.sub.1 and the hydraulic oil in a cylinder chamber C.sub.2 is discharged into the oil tank 64, so that the piston P shifts to the right side and the water in the plunger chamber C.sub.4 is pressurized by the plunger P.sub.2 to be boosted in proportion to the ratio of sectional area of the piston P to the plunger P.sub.2. The water which is boosted by the booster 41 to superhigh pressure is ejected from the jet nozzle 52 to the material 56 to be cut after passing through the check valve 48, accumulator 50, and the nozzle on-off valve 51 at symbol position V.sub.11. Whilst, water from the water supply pump 45 is sucked via the check valve 43 into the plunger chamber C.sub.3, as the pressure therein turns negative as a result of the shifting of the piston P to the right.
Subsequently, when the two-position directional control valve 54 is switched to symbol position V.sub.2, the hydraulic oil from the hydraulic pump 53 is supplied to the cylinder chamber C.sub.2 and the piston P is shifted to the left, so that the water in the plunger chamber C.sub.3 is pressurized by the plunger P.sub.1. Thus, the boosted water or superhigh pressure water is similarly ejected to the material 56 to be cut via the check valve 47 and other associated members. Whilst, water from the water supply pump 45 is sucked into the plunger chamber C.sub.4 which is now under negative pressure.
When superhigh pressure water is ejected from the jet nozzle 52 in this way to cut the material 56 to be cut on the moving carriage 55 while moving the carriage in the direction of arrow X, the on-off valve 59 is opened by exciting a solenoid S.sub.1, and air supplied from a pneumatic source 61 is ejected from an air nozzle 57 to blow away naps, dust and water deposits present on a cut surface just after cutting. For the purpose of cutting while moving the moving carriage 55 in the direction of arrow Y, the on-off valve 60 at the opposite side is opened by exciting a solenoid S.sub.2 and air is ejected from a pneumatic nozzle 58 to blow away dust and the like for quality improvement with respect to cut surfaces.
FIG. 5 is a graph showing time changes in the strokes of the prior art double-acting hydraulic cylinder 42, wherein a solid line represents strokes of one plunger P.sub.1, while a broken line represents strokes of the other plunger P.sub.2. As may be understood from FIGS. 4 and 5, when the other plunger P.sub.2 is in a rightward ascending press stroke, the one plunger P.sub.1 is in a rightward descending suction stroke, and simultaneously upon the other plunger P.sub.2 having reached the end of the press stroke for being switched to a rightward descending suction stroke, the one plunger P.sub.1 reaches the end of the suction stroke and is switched to a rightward climbing press stroke. Therefore, at the time of stroke changing, the water pressure in one plunger chamber C.sub.3 is still low which has just come into a press stroke at the end of superhigh pressure water discharge from the other plunger chamber C.sub.4 into the water discharge line 49 which had reached the end of a pressing stroke thereof, so that if the condition remains as such, there will occur an abrupt decrease in the water pressure of the water discharge line 49 which will result in considerable fluctuations in the water pressure of the water discharge line 49.
Thus, in order to alleviate such fluctuations in the discharge water pressure, a superhigh-pressure type accumulator 50 is disposed on the water discharge line 49 at the downstream of the discharge check valves 47, 48, whereby the pulsation of the superhigh pressure water is attenuated to permit smooth supply to the jet nozzle 52.
However, this poses a problem that the accumulator 50 is very expensive to produce because it is for superhigh pressure service and, in addition, is required to have a considerable volume if pulsation is to be eliminated to an extent sufficient to improve the performance of the booster 41 and the service life of various components used in the oil hydraulic and water hydraulic circuits, which results in considerable increase in the size of the booster and in the cost of booster production.